The use of zeolite adsorbents containing zeolites of NaY and/or NaLiY type for selectively adsorbing meta-xylene in a mixture of aromatic hydrocarbons is well known in the prior art.
For example, U.S. Pat. No. 4,306,107, U.S. Pat. No. 4,326,092, U.S. Pat. No. 5,382,747, U.S. Pat. No. 5,900,523 and U.S. Pat. No. 7,728,187 and also FR 2 889 698 and FR 2 889 699 show that zeolite adsorbents comprising aluminosilicates based on sodium or based on sodium and lithium are effective for separating out the meta-xylene present in aromatic C8 fractions (fractions comprising aromatic hydrocarbons containing 8 carbon atoms).
The adsorbents described in U.S. Pat. No. 5,900,523 are used as adsorption agents in liquid-phase processes, preferably of simulated counter-current type, similar to those described in U.S. Pat. No. 2,985,589 and which apply, inter alia, to aromatic C8 fractions.
In the patents listed above, the zeolite adsorbents are present in the form of crystals in powder form or in the form of agglomerates predominantly formed from zeolite powder and up to 20% by weight of inert binder.
The synthesis of FAU zeolites is usually performed by nucleation and crystallization of silico-aluminate gels. This synthesis leads to crystals (generally in powder form), which are particularly difficult to use on an industrial scale (substantial losses of feedstocks during the manipulations). It is thus preferred to use the agglomerated forms of these crystals, in the form of grains, strands and other agglomerates, these said forms possibly being obtained by extrusion, pelletizing, atomization and other agglomeration techniques known to those skilled in the art. These agglomerates do not have the drawbacks inherent in pulverulent materials.
Moreover, zeolite crystals are usually prepared from aqueous sodium solutions (for example aqueous sodium hydroxide solution), and, if so desired, the sodium cations may be totally or partly replaced (exchanged) with other cations, for example lithium. These cation exchanges may be performed before and/or after agglomeration of the pulverulent zeolite with the agglomeration binder, according to standard techniques known to those skilled in the art.
The agglomerates, whether in the form of platelets, beads, extrudates or the like, are generally formed from zeolite crystals, which constitute the active component (as regards adsorption) and an agglomeration binder. This agglomeration binder is intended to ensure the cohesion of the crystals with each other in the agglomerated structure, but must also be able to ensure sufficient mechanical strength for said agglomerates so as to avoid, or at the very least to minimize, the risks of fracturing, breaking or cracking that might arise during their industrial use during which the agglomerates are subjected to numerous stresses, such as vibrations, large and/or frequent pressure variations, movements and the like.
The preparation of these agglomerates is performed, for example, by slurrying zeolite crystals in powder form with a clayey paste, in proportions of the order of 80% to 90% by weight of zeolite powder per 20% to 10% by weight of binder, followed by forming into beads, platelets or extrudates, and heat treatment at high temperature to bake the clay and to reactivate the zeolite, the cation exchange(s), for instance the total or partial exchange with lithium, possibly being performed before and/or after agglomeration of the pulverulent zeolite with the binder.
Zeolite substances are obtained, the particle size of which is a few millimeters, or even of the order of a millimeter, and which, if the choice of the agglomeration binder and the granulation are performed in a standard manner, have a satisfactory set of properties, in particular of porosity, mechanical strength, abrasion resistance, and the like. However, the adsorption properties of these agglomerates are obviously reduced relative to the starting active powder on account of the presence of agglomeration binder which is inert with respect to adsorption.
Various means have already been proposed for overcoming this drawback of the agglomeration binder being inert as regards the adsorption performance, among which is the transformation of all or of at least some of the agglomeration binder into zeolite that is active as regards adsorption. This operation is now well known to those skilled in the art, for example under the name “zeolitization”. To perform this operation, zeolitizable binders are used, usually belonging to the kaolinite family, and preferably calcined beforehand at temperatures generally between 500° C. and 700° C.
U.S. Pat. No. 4,306,107 describes the use, as a selective adsorbent for meta-xylene, of a zeolite Y, in which the exchangeable cationic sites are occupied by sodium atoms. To obtain satisfactory selectivity in favor of meta-xylene, it is recommended to use a partially hydrated zeolite which has a loss on ignition at 950° C. of from 2% to 7% by weight relative to the initial weight of the adsorbent. Said document recommends separation via a simulated moving bed process at a temperature between 20° C. and 250° C. and at a pressure between atmospheric pressure and 3.5 MPa (35 bar), this value being chosen so as to keep the feedstock in liquid form. The chosen desorbent is toluene. Occupation of the exchangeable sites of the zeolite with sodium ions and activation so as to obtain the desired loss on ignition make it possible to obtain agglomerates which have, from the point of view of adsorption of the meta-xylene contained in aromatic C8 fractions, improved capacity and selectivity properties.
Besides a high adsorption capacity and good selectivity properties toward the species to be separated from the reaction mixture, the adsorbent must have good matter transfer properties so as to ensure a sufficient number of theoretical plates to achieve efficient separation of the species in the mixture, as indicated by Ruthven in the book entitled “Principles of Adsorption and Adsorption Processes”, John Wiley & Sons, (1984), pages 326 and 407. Ruthven indicates (ibid., page 243), that, in the case of an agglomerated adsorbent, the overall matter transfer depends on the addition of the intra-crystalline diffusional resistance and the inter-crystalline diffusional resistance.
The intra-crystalline diffusional resistance is proportional to the square of the diameters of the crystals and inversely proportional to the intra-crystalline diffusivity of the molecules to be separated.
The inter-crystalline diffusional resistance (also known as the “micropore resistance”) is, itself, proportional to the square the diameters of the agglomerates, inversely proportional to the porosity contained in the macropores and mesopores (i.e. the pores whose diameter is greater than 2 nm) in the agglomerate, and inversely proportional to the diffusivity of the molecules to be separated in this porosity.
The size of the agglomerated adsorbents is an important parameter during the use of the adsorbent in industrial application, since it determines the loss of feedstock in the industrial unit and the uniformity of filling. The particle size distribution of the agglomerates must thus be narrow, and centered on number-mean diameters typically between 0.40 mm and 0.65 mm so as to avoid excessive losses of feedstock. The porosity contained in the macropores and mesopores does not participate in the adsorption capacity. Consequently, a person skilled in the art will not seek to increase it for the purpose of reducing the macropore diffusional resistance, given that this would take place at the expense of the volume-based adsorption capacity.
To estimate the improvement in the transfer kinetics, use may be made of the plate theory described by Ruthven in “Principles of Adsorption and Adsorption Processes”, ibid., pages 248-250. This approach is based on the representation of a column by a finite number of ideally stirred hypothetical reactors (theoretical stages). The equivalent height of theoretical plates is a direct measurement of the axial dispersion and of the resistance to matter transfer of the system.
For a given zeolite structure, a given size of adsorbent and a given operating temperature, the diffusivities are fixed, and one of the means for improving the matter transfer consists in reducing the diameter of the crystals. A gain in the overall transfer will thus be obtained by reducing the size of the crystals.
A person skilled in the art will thus seek to minimize the diameter of the zeolite crystals in order to improve the matter transfer.
U.S. Pat. No. 7,728,187 thus claims a process for separating meta-xylene from a mixture of aromatic C8 hydrocarbons which consists in placing the mixture in contact, under adsorption conditions, with an adsorbent, in which the crystals of zeolite Y exchanged with sodium are between 0.05 μm and 0.7 μm in size, so as to improve the matter transfer performance.
The Applicant has nevertheless observed that the synthesis, filtration, manipulation and agglomeration of zeolite crystals which are less than 0.7 μm in size involve cumbersome, sparingly economic processes which are thus difficult to industrialize. In addition, such adsorbents comprising crystals less than 0.7 μm in size also prove to be more fragile, and it then becomes necessary to increase the content of agglomeration binder in order to reinforce the cohesion of the crystals with each other in the agglomerate.
However, increasing the content of agglomeration binder leads to densification of the agglomerates, which is the cause of an increase in the macropore diffusional resistance. Thus, despite a reduced intra-crystalline diffusional resistance due to the decrease in the size of the crystals, the increase in the macropore diffusional resistance on account of the densification of the agglomerate does not allow an improvement in the overall matter transfer. In the context of the present invention, the term “agglomerate” may be used in place of the term “adsorbent” and should be understood as defining the same object.
There is consequently still a need for zeolite adsorbent materials prepared from zeolite of FAU type that are easily manipulable at the industrial level, i.e. whose constituent crystals are advantageously greater than 0.7 μm in size, but having an improved overall matter transfer relative to that of an adsorbent prepared from conventional zeolite crystals of FAU type of identical size (i.e. greater than 0.7 μm), while at the same time conserving high adsorption capacity.
Such zeolite adsorbent materials based on zeolite of FAU type, and in particular containing zeolites of NaY and/or NaLiY type, might provide significant improvements to processes for separating xylenes present in the form of mixtures of isomers in aromatic C8 hydrocarbon fractions, and in particular might greatly improve the selective adsorption of meta-xylene in mixtures of aromatic C8 hydrocarbons.